1. Field of the Invention
The present invention pertains to acoustic logging tool transducers particularly adapted as signal receiving transducers which utilize piezoelectric or piezoresistive elements.
2. Backqround
Several acoustic logging tools have been developed with a view to improving the sensitivity of the signal-transmitting and -receiving transducers and with a view to developing tools which are capable of sending and receiving compressional waves and so-called shear waves. U.S. patent applications Ser. No. 07/300,010, filed Jan. 23, 1989 in the name of Keith W. Katahara et al, Ser. No. 07/319,607, filed Mar. 6, 1989 in the name of Keith W. Katahara, Serial No. 07/353,067 filed May 17, 1989 in the name of Steven G. Petermann et al, and Ser. No. 07/363,660, filed June 8, 1989 in the name of Keith W. Katahara, all assigned to the assignee of the present invention, disclose certain piezoelectric actuators and transducers for use with acoustic logging tools. Certain references are cited by the applicant in these applications and which comprise the prior art of which the Applicant in this application is aware and which may be considered to be relevant to the subject matter of this application. These references are U.S. Pat. Nos. 3,593,255; 4,319,345; 4,649,525; 4,649,526; 4,682,308; 4,715,019; 4,718,046; British Patent Applications Nos. 2,122,351 and 2,124,377 to Exxon Production Research Company; and European Patent Publication No. 0,246,773 to Mobil Oil Corporation.
The above-mentioned patent applications describe various types of transducers including piezoelectric types which are particularly useful as signal-transmitting devices for imparting an acoustic wave into a fluid-filled borehole for transmission to and through the earth formation surrounding the borehole and whereupon return signals from the earth formation are sensed by transducers in a logging tool which may be similar to the transducers disclosed in the referenced applications. However, generally speaking, the acoustic logging tools and transducers described in the aforementioned applications are suited more for signal transmitting or signal generating devices than for signal receiving devices. In view of the energy losses from pressure wave signals when transmitted through borehole fluids and earth formations, the sensitivity of the receiving transducer is particularly critical. It is toward this end that the present invention has been developed with a view to providing signal-receiving transducers which offer superior sensitivities, thus being able to detect small amplitude borehole acoustic waves, and further, in particular, so-called shear-type acoustic waves.